1. Field of the Invention
The invention relates to a formed part for a vehicle body structural member that makes up part of a vehicle body. More particularly, the invention relates to a formed part for a vehicle body structural member, which is formed from a so-called high strength steel sheet which has a higher tensile strength than does a normal steel sheet, or a so-called ultra high strength steel sheet which has an even higher tensile strength than does a high strength steel sheet.
2. Description of the Related Art
Some vehicle body structural members such as roof side rail outer reinforcements provided along the roof side of a vehicle body, and center pillar upper reinforcements and center pillar lower reinforcements provided along the center pillar of the vehicle body are used which have hat-shaped cross-sections that open in the vehicle width direction. Moreover, such vehicle body structural members having this kind of structure are formed by drawing (i.e., press-forming) sheets of steel (Japanese Patent Application Publication No. 2006-281312 (JP-A-2006-281312) describes one such example).
In recent years, these kinds of vehicle body structural members are being made of so-called high strength steel sheets which have a higher tensile strength than normal steel sheets. Further, ultra high strength steel which has an even higher tensile strength than a high strength steel sheet also is being considered for making such vehicle body structural members.
On the other hand, roof sides are often curved downward at the front so that the front is lower than the center in the longitudinal direction of the vehicle. When a roof side rail outer reinforcement having a hat-shaped cross-section is formed following that shape, the entire roof side rail outer reinforcement curves so that the center of curvature is positioned on one side in the width direction of the headwall portion (i.e., referred to as the hat-shaped head section in JP-A-2006-281312).
When a flat sheet of steel is drawn (press-formed), the linear length in the longitudinal direction of the headwall portion is different before and after forming at a vertical wall portion formed continuous from the edge portion in the width direction of the headwall portion. This difference in linear length (also hereinafter referred to as “linear length difference”) results in tensile stress in the longitudinal direction of the headwall at the vertical wall portion on a first side of the headwall portion in the width direction (i.e., on the side on which the center of curvature of the curve is located). In addition and conversely, compression stress in the longitudinal direction of the headwall is generated in the vertical wall portion on a second side of the headwall portion in the width direction (i.e., on the side opposite the side on which the center of curvature of the curve is located). When this stress is generated in the vertical wall portion, it causes the vertical wall portion to lean and twist above its axis which lies in the longitudinal direction of the head wall portion.
This kind of twisting is significant when high strength steel sheets, which have a higher tensile strength than normal steel sheets, are used, and even more significant when ultra high strength steel sheets, which have an even higher tensile strength than high strength steel sheets, are used, making it difficult to obtain the desired shape.